1. Field of the Invention
The present invention relates to a semiconductor device which includes an oxide semiconductor film.
In this specification, the semiconductor device refers to all the elements and the devices that operate by utilizing semiconductor characteristics. A typical example of the element is a transistor. A display device, a semiconductor circuit, and an electric device each of which includes the transistor are included in the semiconductor device in its category.
2. Description of the Related Art
A transistor has switching characteristics and therefore is used as a switching element of a pixel in a liquid crystal display device, a light-emitting display device, or the like. The switching characteristics of the transistor are utilized not only in a liquid crystal display device or a light-emitting display device but also in a semiconductor memory element.
For example, a dynamic random access memory (DRAM) which is one of semiconductor memory devices has a memory cell including a transistor and a capacitor. The transistor needs to be miniaturized for the purpose of reduction of the memory cell size in order to improve the integration degree of the semiconductor integration circuit without an increase in size of chips in the DRAM.
When a transistor is miniaturized, the channel length thereof is extremely shortened. In such a case, fluctuation in electric characteristics of the transistor such as the negative shift in the threshold voltage might occur. When a transistor is miniaturized, it is important to suppress this phenomenon called a short-channel effect to miniaturization of a transistor.
Because of the short-channel effect, a leakage current due to a punch-through phenomenon is likely to flow in a transistor whose channel length is extremely shortened; as a result, the transistor does not function as a switching element in some cases. In order to prevent the leakage current from flowing, a silicon substrate may be doped with an impurity at high concentration. However, this is not an appropriate solution to the problem because it makes a junction leakage current likely to flow between a source and the silicon substrate or between a drain and the silicon substrate and eventually causes a deterioration of memory retention characteristics.
Against such a problem, a method has been considered for maintaining an effective channel length long enough by forming a three-dimensional transistor in the memory cell in order to prevent a short-channel effect. An example is a structure in which a U-shaped vertically long groove is formed in a region where a channel portion of a transistor is formed, a gate insulating film is formed along a wall surface in the vertically long groove, and a gate electrode is formed so as to fill the vertically long groove (see Non-Patent Document 1).
In a transistor with such a structure in the channel portion, current which flows from a source to a drain via the vertically long groove portion; therefore, an effective channel length is longer than an apparent channel length which is a distance between the source electrode and the drain electrode in the transistor seen from the top. In this manner, the transistor can be reduced in size and the short-channel effect thereof can be suppressed.